This invention relates to a method for analyzing the consequences (both relative to economics and relative to plant metrics) of employing different alternative strategies for managing the operation of large heat exchangers, e.g., those with more than about 10,000 square feet (900 square meters) of heat transfer surface area. Heat exchangers serve as a device for transferring heat from one medium to another.
Large heat exchangers such as recirculating nuclear steam generators typically comprise the following major components: a) an outer (typically vertically oriented) shell, b) a plurality of tubes, which are often disposed in an inverted U configuration, that collectively form a tube bundle located within the outer shell, c) a cylindrical plate known as the wrapper which is located between the outer shell and the tube bundle and serves to direct incoming liquid flowing into the steam generator, d) a thick plate known as the tubesheet which is connected on one side to, and penetrated by, each of the tubes in the tube bundle and separates an upper secondary side of the heat exchanger from a lower primary side of the heat exchanger, e) a plurality of thinner plates spaced periodically along the lengths of the inverted U-tubes, known as tube support plates, that provide structural support to the inverted U-tubes during operation of the steam generator, f) a divided chamber known as the primary channel head which is attached to the other side of the tube sheet and contains both an entrance plenum and an exit plenum that are in communication through the plurality of inverted U-tubes, g) moisture separation components located above the tube bundle that might include cyclone separator units, hook-and-vane dryers, or other components that separate liquid from steam, and h) a steam outlet nozzle from which steam produced within the steam generator exits the steam generator. Some steam generators also contain: i) a sub-assembly of pipes arranged in a circle located above the tube bundle and known as the feedring which is used to inject liquid water into the steam generator, and/or j) a sub-assembly of plates arranged within the tube bundle, which are known collectively as a preheater or economizer, into which liquid water enters the steam generator.
The basic functioning of a recirculating nuclear steam generator involves heating of a secondary fluid by a primary fluid. The primary fluid's path through the steam generator is described by the following sequence: 1) primary fluid is heated by circulation through the core of the nuclear reactor and then enters the steam generator through the entrance plenum in the primary channel head, 2) this primary fluid then enters the insides of the inverted U-shaped tubes at the lower (primary) face of the tube sheet, to which the inverted U-tubes are attached, 3) the primary fluid is carried through the full length of inverted U-tubes, heating the U-tubes and, through the heated tubes, the secondary fluid present on the outside of the U-tubes, 4) the primary fluid exits the U-tubes through the tube sheet into the exit plenum of the primary channel head, and 5) the primary fluid exits the steam generator through an outlet nozzle in the primary channel head, after which it is returned to the nuclear reactor for reheating.
The secondary fluid's path through the recirculating steam generator is typically described by the following sequence: 1) secondary fluid enters the steam generator as a liquid through an injection nozzle into a feedring above the tube bundle or, alternatively, directly into the preheater, 2) the secondary fluid then either enters the annular space (known as the downcomer) between the outer shell and the wrapper or, alternatively, proceeds through the preheater, 3) the secondary fluid exits the downcomer or the preheater at or near the upper surface of the tube sheet, 4) the secondary fluid then flows upward through the tube bundle, in contact with the outsides of the inverted U-tubes where it is heated by the U-tubes, 5) during its upward journey, the secondary fluid boils to produce a two-phase mixture of steam and liquid water, and 6) after exiting the tube bundle at the top, the secondary fluid enters moisture separation equipment which segregates the secondary fluid into its liquid component, which is recycled into the downcomer, and its steam component, which exits the steam generator through an outlet nozzle disposed at the uppermost end of the outer shell. The steam portion of the secondary fluid passes through standard electrical generating equipment, including turbines and a condenser, before returning to the steam generator in liquid form for reheating.
Large heat exchangers other than recirculating nuclear steam generators may have different basic components and component arrangements than those described in the above paragraphs (e.g., helical, plate-frame, and compact heat-exchanger designs such as printed-circuit heat exchangers in gas-cooled reactors). Similarly, the descriptions of degradation modes in the paragraphs below are particular to recirculating nuclear steam generators. However, the application of the invention is not limited to these heat exchangers or the particular degradation modes described.
Normal operation of nuclear plants leads to the flow of large masses of secondary fluid through the recirculating nuclear steam generator tube bundles (typically on the order of several million pounds of water per hour per steam generator). Inevitably, small concentrations of impurities such as iron oxide and copper originating from metallic plant components exterior to the steam generators contaminate the secondary fluid. Although these impurities are present in very small concentrations (on the order of one part per billion), the large flow rates of secondary fluid through the steam generator ensure that significant quantities (on the order of one hundred pounds or more) of impurities enter each operating steam generator during each year of operation. The majority (in excess of 50% and often more than 90%) of these impurities deposit within the steam generator, with the largest fraction thereof depositing as scale layers on the exterior surfaces of the inverted U-tubes and a typically smaller fraction settling on the top of the tube sheet surface where it often consolidates into a hardened “sludge pile”. The tube-surface deposits can after a period of time lead to a decrease in the heat-transfer efficiency of the steam generator, a process known as tube deposit heat-transfer fouling. This fouling generally reduces the thermal efficiency of the entire plant, lowering the electrical power which is produced. In some cases, the reduction in plant output can be substantial (several percent or more) unless remedial actions are taken.
In addition to deposit heat-transfer fouling, most nuclear steam generators in operation are susceptible to service-induced corrosion and wear of the inverted U-tubes through a number of distinct mechanisms. The corrosion mechanisms can initiate on the inner (primary) tube surface or on the outer (secondary) tube surface. Corrosion that initiates on the outer tube surfaces has been shown in some circumstances to be exacerbated by the presence of deposits on tube surfaces and deposits within crevices formed by the U-tubes at the locations where they pass through the tube sheet and tube support plates. Because the U-tubes serve as a structural boundary between the primary fluid, which circulates through the reactor, and the secondary fluid, most types of corrosion require that the affected tubes be repaired (e.g., through installation of a protective sleeve attached to the inside surface of the tube that permits the tube to remain in service) or removed from service (e.g., through plugging of each end, preventing flow of primary fluid through the tube) as soon as such corrosion is detected through routine inspection methods. Removal of tubes lowers the heat-transfer capability of the steam generator, reducing plant output in a way analogous to that associated with tube deposit heat-transfer fouling. Typically, steam generators with susceptible tubing will experience corrosion of an increasing fraction of the tube bundle as operating time accumulates. Eventually, if a sufficient number of tubes is removed from service, the steam generator must be entirely replaced to permit continued plant operation.
In a nuclear steam generator, each tube support plate typically contains an array of holes therein for accommodating passage of the U-shaped tubes through the tube support plates. The height of the U-shaped tubes may exceed 30 feet (9 m), and a steam generator therefore typically includes six or more tube support plates, each horizontally disposed along the vertically oriented tube path, with adjacent tube support plates typically having a vertical separation of 3 to 5 feet (0.9 to 1.5 m). Tube support plates may comprise solid metallic plates with machined openings that may be circular in shape (“drilled holes”) or lobed in shape (“broached holes” with three lobes (“trefoil” design) or four lobes (“quatrefoil” design) being common). In other designs, the tube support plates may comprise interlocking arrangements of steel bars known as lattice bars.
For tube support plates with drilled holes, the inverted U-tubes pass through some of the circular holes within the tube support plates while secondary fluid passes through other such circular holes. Thus, the large majority of the secondary fluid does not pass through the circular holes through which the U-tubes pass. In contrast, for tube support plates with broached holes, the secondary fluid passes through the lobes and therefore comes into contact with the heated tubes while it is passing through the tube support plates. In a number of cases, steam generators with broached hole tube support plates have experienced service-induced blockages of the lobes through which the secondary fluid passes during operation of the steam generator. These blockages occur as a result of deposition of corrosion products that are suspended and/or dissolved within the secondary fluid onto the lobe surfaces. In severe cases, many lobes can become fully blocked, admitting no secondary fluid flow and causing controllability problems that require the plant power level to be decreased or the plant to be shut down until the blocking deposits can be partially or fully removed.